1. Field of the Invention
The present invention relates to modifying a waste heat exchange device in cooling exhaust gases for the purposes of removal of elemental mercury from the exhaust gases using chemi-deposition (heterogeneous (gas/surface) interactions).
2. Description of the Prior Art
Many American states and the U.S. Environmental Protection Agency are on the point of legislating mercury emission controls in the very near future. As a result, many testing programs have been underway in recent years to provide practical methods that might apply primarily to the coal industry, and possibly also to incineration, cement plants and metal smelting industries that also encounter this problem. Although many suggested methods have been proposed, few have been found to be adequate for the practical demands of full-scale industry.
Currently the two major contending methods that are being recommended by organizations such as The Electric Power Research Institute and the U.S. Department of Energy are those of invoking activated charcoal injection into the cool flue gases or alternatively to mix bromine in some form with the fuel. The concept of using activated charcoal arose from the known fact that in coal combustion a small fraction of the mercury is found on the resulting fly ash. Attempts to manipulate the nature of the fly ash itself to enhance its efficiency have seen only limited success. Efficiencies appear to be roughly correlated to the amount of unburned carbon in the fly ash. However, companies are reluctant to reduce the efficiencies of their combustors so as to produce ash that in essence has not fully burned. Moreover, fly ash often is sold as an ingredient of concrete, but can only have about 4% carbon content otherwise becomes unacceptable. As a result, emphasis has been in the development of activated charcoal addition by spraying it as fine particles into the cool flue gases downstream. However, this is a very inefficient interaction that is still not understood except that the mercury does appear chemically and stably absorbed. Nevertheless, many thousands of carbon atoms have to be used to remove one atom of mercury. As a result of such low efficiencies, enhancements have been developed using pre-treated activated charcoals. These are available and can contain sulfur, chlorine or bromine doping. The method can be effective but remains expensive and labor intensive. If fly ash sales are involved, it requires two particle removal systems, one for the fly ash followed by one for the activated charcoal. Also, there always remains the possibility of these wastes becoming defined as toxic materials which would elevate costs.
A second method shown in B. Vosteen et al., “Process for Removing Mercury from Flue Gases,” U.S. Pat. No. 6,878,358, April, 2005, incorporated herein by reference, is very elegant in having discovered that additions of bromine can enhance conversion of the atomic mercury. Gaseous mercury dibromide is produced that is water soluble and can be removed by sulfur scrubbers currently in widespread use. Bromine is seen to be much more efficient than chlorine in such interactions. Its limitation however is that bromine or its salts are far less abundant than chlorine and remain rather expensive introducing a financial hurdle for wide general usage.
Other methods that are known invoke the presence of high levels of chlorine in the system. This can be achieved either by additions of chlorine or by blending fuels one of which is very rich in chlorine. Again this can significantly elevate efficiencies but chlorine is a highly corrosive element and detrimental to combustor systems. The Selective Catalytic Reducer (SCR) now being utilized in certain plants for reducing nitric oxide (NO) to the regulated allowable emission levels contains many catalytic surfaces and generally operates in a temperature range of about 340-380° C. (640-710° F.). Because of previous work that supported the use of catalysts to convert atomic mercury, it was hoped that the SCR could be made to do double duty and also reduce mercury emissions. However, after extensive testing it now appears that only limited conversions can be achieved. The difficulties are that the temperatures in the SCR are too high for mercury conversion and also the required addition of ammonia needed to reduce NO are contrary to the conditions sought by the mercury. The optimization of either causes a detriment to the other.
Many other patents have been published in the past based on absorbing atomic mercury by its amalgamation or catalytic properties. These have all been considered and some tested but none found to be satisfactorily practical for industrial large scale applications.
As a result, although the prior art in mercury control methods has examined this problem in depth it has failed in creating a practical low cost device by which mercury emissions can be controlled especially for application to the coal industry. Coal remains a major global fuel and as a result requires a simple method that can be invoked globally without significant cost. The present method of redesigning the air preheater device has never been considered before. Preliminary pilot-plant testing under realistic coal combustion conditions now has validated the concept. It supports the practical application of such a modification to this well known piece of equipment that is invariably necessary as a waste heat exchanger in any power plant system. It has not been realized until now that it can do double duty and also efficiently act as a mercury conversion and control device.